1. Field of the Invention
This invention pertains to the field of alkylating isoparaffinic hydrocarbons with olefinic hydrocarbons in the presence of an acid catalyst including the use of effluent refrigeration. More specifically, the present invention relates to the use of enhanced boiling surface on the inside surface area of the chiller tubes within the reaction zone.
2. Discussion of Related Art
Alkylation in the petroleum refining industry involves the chemical reaction of isoparaffinic hydrocarbons with olefinic hydrocarbons, typically isobutane with isobutylene, in the presence of a catalyst such as sulfuric or hydrofluoric acid, to product C.sub.8 branched hydrocarbons such as trimethylpentanes (iso-octanes). These higher molecular weight "alkylates", as they are called, improve the anti-knock properties of motor gasoline. The increased use of unleaded fuels has considerably increased the importance of the alkylation process within the refining industry.
Alkylation has been commercially practiced for more than fifty years, and is competitive with other octane enhancement processes when there is a ready supply of isobutane and isobutylene. Generally, there are four different methods for carrying out the alkylation process today: sulfuric acid autorefrigeration, sulfuric acid-effluent refrigeration, hydrofluoric acid-time tank, and hydrofluoric acid tubular reactor. Of these four methods, only sulfuric acid-effluent refrigeration involves the boiling of refrigerant within a heat exchanger to cool the reaction zone. It is this method of alkylation with which the present invention is primarily directed. Reference is made to U.S. Pat. Nos. 2,664,452, 2,906,796 and 2,949,494 which describe such an alkylation technique and which are incorporated herein by reference.
The basic chemical reaction for isobutane and isobutylene in the presence of concentrated sulfuric acid at 40.degree. to 55.degree. F. is the following: EQU C.sub.4 H.sub.10 +C.sub.4 H.sub.8 &gt;C.sub.8 H.sub.18 (iso-octane)
Generally, in the basic process, concentrated sulfuric acid is mixed with cracked gases containing olefinic components such as propylene and isobutylene as well as propane and butane in addition to both fresh and recycled isobutane in the reaction zone, with about 40% sulfuric acid being present by volume. The fluids are not miscible, and the C.sub.4 fractions float on the acid. When the reactor is vigorously agitated, the hydrocarbons break up into extremely fine droplets, and an emulsion is formed which increases the rate of reaction. After about 1 hour at 45.degree. F. and 70 psia, a yield of about 15% to 20% alkylate is obtained.
Temperature is a critical variable in the alkylation reaction. The lower the temperature, the less the tendency to form undesirable side reactions involving self alkylation of the isobutane, or reaction of the acid to form alkyl sulfates. Consequently, better quality alkylate and more conversion is obtained when the reaction is carried out at the lower temperature, with the lower limit being generally about 35.degree. F. This lower limit is primarily set by the high viscosity of the acid at that temperature and the cost of the refrigeration reguired to remove the exothermic heat of reaction.
Since temperature is so important in the alkylation reaction and in view of the fact that the reaction is exothermic in nature, it is therefore necessary to continually remove the heat of reaction to maintain a desirable temperature of reaction. In order to do so, the sulfuric acid effluent refrigeration technique of alkylation involves continuously passing effluent from the reactor to a settler (while the reaction is going on) to separate the effluent into a hydrocarbon phase and an acid catalyst phase. The hydrocarbon phase is then reduced in pressure thereby lowering its temperature. This cooled stream, now containing both liquid and flashed vapor created by the pressure reduction, is then passed in indirect heat exchange relationship through the reaction zone thereby removing the heat of reaction. Typically, the cooled hydrocarbon stream passes through a U-shaped tube bundle chiller which is provided within the reaction zone. As the heat of reaction is transferred to the chiller, more of the liquid hydrocarbon vaporizes inside the chiller tubes.
The large amount of vapor generated by the heat of reaction in the hydrocarbon stream is separated from the liquid portion of the stream, typically in a suction trap. The liquid goes to a fractionation step, while the vapor passes through a compressor and condenser to form a further liquid phase. This further liquid phase is then throttled to an intermediate flash tank which is at the same pressure as the suction trap, both pressures being controlled by the suction pressure on the compressor. Flashed vapors are recycled to the compressor, while the liquid phase, known commonly in the art as "effluent refrigerant" and consisting primarily of isobutane, is recycled back to the reaction zone providing additional cooling of the reaction.
While the alkylation reaction is most desirably carried out at the optimum temperature range of from about 40.degree. F. to 45.degree. F., most commercial plants generally operate their alkylation reactors at the higher temperature of 50.degree. F., and in some cases as high as 55.degree. F. or higher, due to economical and safety factors.
More particularly, referring to the well known heat transfer equation: EQU Q =U.times.A.times.(T.sub.reactor -T.sub.boiling fluid)
where Q is the heat generated by the heat of reaction in BTU/hr; U is the overall heat transfer coefficient in units of (BTU/hr)/(ft.sup.2 .degree.F.); A is the surface area of the chiller in ft.sup.2 ; T.sub.reactor is the temperature of the reaction; and T.sub.boiling fluid is the mean temperature of coolant hydrocarbon phase; it is seen that for a given heat duty Q, there are essentially four different parameters that can theoretically be varied in order to provide the necessary heat transfer.
Of all of these parameters, increasing the surface area of the chillers involves the most costly option. Due to practical design limitations of the chiller/reactor units, commonly known in the art as "contactors", a plurality of these contactors must be provided if a substantial amount of chiller surface area is needed. Aside from the capital costs involved in providing such additional contactors, there are a number of factors which come into play which add to the disadvantages of this approach. The first effect is that additional contactors each employ an additional agitator within each respective reaction zone. This now undesirably adds mechanical energy into the reaction zone which increases the heat load of the system. Of course, the costs associated in operating these additional contactors also increases. Secondly, the reactor space velocity decreases when employing a plurality of contactors, typically in parallel. This tends to increase the production of high boiling components, such as alkylates, within the reaction effluent which undesirably elevates the boiling temperature and reduces the temperature difference available for heat transfer. Finally, inasmuch as the chiller fluid has to pass through a greater number of contactors, the tube side velocity is reduced with a concomitant reduction in the inner tube heat transfer coefficient which in turn leads to a decrease in the overall coefficient. The net result of all of the above is that increasing the chiller surface area is not the most viable alternative.
The next alternative is to attempt to decrease the temperature of the boiling fluid, i.e., the fluid passing through the chiller, so as to provide a greater temperature gradient for the required heat transfer. This approach too is disadvantageous. As noted above, the boiling fluid is obtained by partially flashing the separated hydrocarbon phase coming from the acid settler to a reduced pressure. This reduced pressure is controlled by the suction pressure of the compressor in which the vaporized hydrocarbons within the boiling fluid are ultimately passed. As would be readily apparent to one skilled in the art, the temperature of the boiling fluid entering the chiller bundle is dependent upon the suction pressure of the compressor. By reducing the suction pressure, the temperature of the boiling fluid is correspondingly reduced. However, in order to provide a boiling fluid temperature which is low enough to establish a sufficient temperature gradient, the suction pressure of the compressor would undesirably have to operate under partial vacuum. Thus, if a system were operating at a reaction temperature of about 52.degree. F. with the boiling fluid entering the chiller at 25.degree. F. at a suction pressure of 17 psia, in order to reduce the reaction temperature to 42.degree. F., for example, the chiller temperature would have to enter at a temperature of 15.degree. F. which would require a compressor suction pressure of 13 psia, which is below atmospheric. Moreover, due to the lower boiling fluid temperature and pressure which leads to a decrease in the vapor density, more compressor power would be required to compress the same amount of vapor.
Accordingly, attempting to accommodate the heat duty of the system by reducing the boiling fluid temperature will lead to increased power consumption or require the use of a larger compressor. Most importantly, however, it will also lead to a compressor operating at a suction pressure under vacuum. This condition could lead to the leakage of air into the system and the potentially dangerous buildup of oxygen in the hydrocarbons.
Without simply increasing the reaction temperature in order to provide an increased temperature gradient, the only other variable in the heat transfer equation is U, the overall heat transfer coefficient. This overall heat transfer coefficient is well recognized by those skilled in the art as being dependent upon the combination of the individual liquid film heat transfer coefficient on the outside of the tube which is in contact with the reaction emulsion and the individual boiling film heat transfer coefficient on the inside of the tube which is in contact with the boiling fluid.
Due to the nature of the emulsion, it has been extremely difficult to measure and/or calculate an emulsion heat transfer coefficient for the outside of the chiller tubes. By using well established relationships, it is possible, however, to calculate the heat transfer coefficient for the inner, boiling side of the tube. Having established from existing operating systems that the overall heat transfer coefficient for the chiller in such an alkylation reaction is about 50 to 60 (BTU/hr)/(ft.sup.2 .degree.F.), and after calculating the heat transfer coefficient for the boiling fluid, it is possible to extract a heat transfer coefficient for the emulsion side of the chiller. See, for example, Chen, J. C., "Industrial and Engineering Chemistry, Process Design and Development", Vol. 5, No. 3, pp. 322 (1966) for methods of calculating in tube film coefficients.
Based upon what is conventionally known by one skilled in the art, it is generally believed that the controlling factor influencing the overall heat transfer coefficient is the emulsion heat transfer coefficient on the outside of the chiller. In other words, based on calculations such as that described above, the heat transfer coefficient on the emulsion side generally is believed to be lower than the heat transfer coefficient on the inner boiling side. Consequently, if one skilled in the art were to attempt to increase the overall heat transfer coefficient, he would seek to increase the emulsion side heat transfer coefficient.
However, due to the nature of the emulsion, the prior art has generally refrained from modifying the surface of the outer tube so as to attempt to increase the outside heat transfer coefficient. Thus, a common way of increasing the coefficient would be to add fins to the outer walls of the tubes. However, such extended surfaces would be susceptible to corrosion by the acid; fouling and clogging; and cause a decrease in the flow. As a result, one skilled in the art has stayed away from attempting to modify the overall heat transfer coefficient as a means of providing better heat transfer.
In view of the above, the skilled art worker has found essentially no choice in many cases but to carry out the alkylation reaction at a temperature which is higher than desirable or operate the suction pressure of the compressor as low as possible, even under vacuum, in order to provide the necessary temperature gradient for the required heat transfer. Clearly, a need exists to improve this alkylation process, particularly to be able to run the reaction at the most optimum temperature without the need to run the compressor suction pressure under vacuum.